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Stress relaxation measurements were carried out on a plain carbon and four Ti steels over the temperature range 850 to 1050$ sp circ$C. The results show that the stress relaxation of plain carbon austenite after a 5% prestrain can be described by the relation $ sigma$ = $ sigma sb0$-$ alpha$ln(1 + $ beta$t). By contrast, in the solution-treated Ti steels, relaxation is arrested at the start of precipitation and is resumed when precipitation is complete. As a result, this new mechanical method is suitable for following carbonitride precipitation in microalloyed austenite at hot working temperatures. / PTT diagrams were determined by the present technique for the steels containing 0.05, 0.11, 0.18 and 0.25% Ti. The PTT curves obtained are C shaped for all the steels. The upper parts of these curves are shifted to significantly longer times as the Ti and C concentrations are reduced. By contrast, the positions of the lower arms of the curves are relatively independent of the current values of the solubility product (Ti) (C). This phenomenon is attributed to the catalytic effect of trace amounts of dissolved N on the nucleation rate of Ti(CN) in austenite. / Changes in the size distribution and morphology of the precipitates during relaxation of the 0.25% Ti steel were followed by means of electron microscopy. The cube shaped Ti(CN) precipitates were heterogeneously distributed in either a chain-like or a cell-like manner. Electron microanalysis was additionally carried out to determine the compositions of the Ti carbosulphide-Mn sulphide inclusions. / Finally, the experimental results are compared with the predictions of classical nucleation theory and of the diffusion controlled particle growth theory. Good agreement was obtained between the predictions of the theories and the experimental results. It is demonstrated by means of the thermodynamic analysis of nucleation that the Ti(CN) precipitate/austenite interface is of a semi-coherent nature. It is shown that the interface between a critical nucleus and the matrix can be characterized by the newly introduced coherency loss parameter C.

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Laser processing of NiTi shape memory alloys (SMA) has been identified as having great potential in surface treatment, welding, and novel performance requirement applications. However, discrepancies arise regarding whether laser processing improves or degrades the corrosion performance of NiTi-based SMAs. This is a cause for concern over the reliability of the laser processed surfaces. Prior to full scale implementation, a better understanding of oxide evolution during laser processing is required. The first part of this study concerned a systematic investigation of the surface of Ni-44.2 wt.% Ti SMA after the application of differing amounts of laser pulsing and thus energy input. Specific focus was directed on characterizing local changes in the surface oxide adjacent to laser spots. The sample local electrochemical characteristics were investigated by scanning electrochemical microscopy (SECM). The effect of laser processing on the regeneration kinetics of the redox-active mediator was analyzed through microelectrode current maps and approach curves in the feedback mode. Raman spectroscopy was used to determine the crystallinity of the oxide and potentiodynamic cyclic polarization was used to determine oxide stability. Results showed that for a small number of pulses (i.e. low total energy input) corrosion performance was determined primarily by topographical effects. However, increasing the number of pulses (i.e. higher total energy input) had a significant impact on the stability of the oxide in the heat-affected zone (HAZ) region due to the increase in crystallinity, which dictated where the corrosion initiated. In the second part of this study, post-process surface treatments that could be applied to NiTi SMA after laser processing were systematically investigated. Specific focus was directed at characterizing the crystallinity of the newly formed oxides and the stability across the entire surface (containing laser-processed regions and retained base material). Raman spectroscopy and potentiodynamic cyclic polarization were used for this analysis. Results showed that the post-process surface treatments successfully restored the corrosion performance to pre-laser-processing conditions by eliminating crystallinity in the surface oxide and reducing inhomogeneity across the surface.

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